1. Field of the Invention
The present invention relates to a lens array unit and an image reading device using the lens array unit.
2. Description of the Related Art
Some image reading devices such as scanners and image forming devices such as LED printers are known to use erecting equal-magnification optical systems. Erecting equal-magnification optical systems are capable of reducing the size of devices better than reduction optical systems. In the case of image reading devices, an erecting equal-magnification optical system comprises a line light source, an erecting equal-magnification lens array, and a line image sensor. In the case of image forming devices, an erecting equal-magnification optical system comprises a line light source, an erecting equal-magnification lens array, and a photosensitive drum.
A rod lens array capable of forming an erect equal-magnification image is conventionally used as an erecting equal-magnification lens array in an erecting equal-magnification optical system. Recently, there is proposed a lens array unit formed as a stack of a plurality of transparent lens array plates built such that the optical axes of individual convex lenses are aligned, where each transparent lens array plate includes a systematic arrangement of micro-convex lenses on both surfaces of the plate. Since a lens array unit such as this comprises a stack of lens array plates formed by, for example, injection molding, an erecting equal-magnification lens arrays can be manufactured at a relatively low cost.
A lens array unit lacks a wall for beam separation between adjacent lenses. Therefore, there is a problem of stray light wherein a light beam diagonally incident on the lens array unit travels diagonally inside the plate and enters an adjacent convex lens, creating a ghost image as it leaves the plate.
Patent document No. 1 discloses a technology to address stray light whereby the first lens array facing an object on which an image is formed is fitted with a light shielding member for regulating light traveling from the object.
[patent document No. 1] JP 2005-352429
In the lens array unit described in patent document No. 1, the shielding member is fitted to the first lens array such that the plurality of through holes formed in the shielding member are directly aligned with the front surfaces of the corresponding lenses formed in the first lens array by allowing a projection provided in the first lens array to fit a recess provided in the shielding member
When an assembly of the first lens array and the shielding member is formed by fitting a projection to a recess as in patent document 1, certain allowance in the diameter is required for fitting. This may create displacement between the optical axis of the lens of the first lens array and the optical axis of the through hole of the shielding member and poorer imaging performance.